The use of plane linear diffraction gratings on rotating spinners as optical scanners has become an important technology in several industries, such as laser printers and the like. One form of such scanning devices are known in the industry as holographic spinner devices, and are disclosed in Ih U.S. Pat. No. 3,953,105. In this device, a source of coherent radiation is directed at a surface which is being rotated about an axis such that the radiation is redirected to a focal point locus.
Early on it was recognized that the basic simplicity of a rotating hologram scanner brought with it inherent disadvantages, particularly in the failure to produce truly straight line scans. Heiling U.S. Pat. No. 4,094,576 discloses the use of a lens system to remedy the problems incurred with the use of a strongly curved object surface. Heiling uses a reconstructed wave using a planar reference wave and a spherical wave modified by a cylindrical lens. Goshima U.S. Pat. No. 4,121,883 discloses a scanning device with various optical devices in the path of the beam between the rotating member and the final focal point locus.
Kay U.S. Pat. No. 4,428,643 disclosed the use of a diffraction grating in the optical path of a spot scanning holographic spinner system to provide compensation for wavelength shifts in the laser. The diffraction grating has the same properties as do the gratings formed on the spinner surface. This patent defines certain problems which result from the geometry of holographic scanners using a rotating disc having a plurality of holographically formed lenses or gratings. Identified as problems from these devices are scan line "bow", spinner wobble and spinner "wedge". A relationship between wavelength of the reconstructed light source, the grating period and the angles of incidence and diffraction is discussed, as this patent is directed at the need to compensate for wavelength shifts in the coherent light source.
Referenced in the above discussed Kay patent is the disclosure which has become Kramer U.S. Pat. No. 4,289,371. This basic patent discloses the use of a scanning system in which a reconstruction beam is directed in non-normal incidence against a spinner surface having formed therein at least one plane linear diffraction grating. The grating is constructed such that it conforms to a ratio of the wavelength of the reconstruction beam to the grating period (.lambda..sub.r /d) of between 1 and 1.618. As the spinner is rotated about an axis of rotation, the device generates a wavefront or scan line which is focused with the use of other optical elements on an imaging plane.
Kramer U.S. Pat. No. 4,289,371 suggests that such a system produces line scans at a plane which are almost completely bow free and are invariant with respect to any irregularities such as wobble on the surfaces of the spinner. Minimization of bow is described with extensive mathematical analysis and it is disclosed that satisfactory results are achieved. A bow of 0.0016 inch (theoretical data only) is shown in FIG. 10 and is stated to be more than acceptable for most high resolution scanning applications. Actual experimental values are shown in FIG. 11, resulting in bow of as much as between 0.003 and 0.004 inch.
The above discussed patent does not solve all of the bow problems of spot scanning hologon spinner systems. Kramer U.S. Pat. No. 4,583,816 acknowledges that scan line bow is still a problem to be solved and proposes to solve that problem by an improved system. The new Kramer system suggests that bow can be corrected by tilting the lens which is usually employed in cooperation with the hologon deflector to produce the scanning spot. This geometric embodiment places the optical axis of the lens in an axis which is not coaxial with the diffracted beam but is, instead, at an angle with the path of the diffracted beam. The angle is suggested to be sufficient to bend the scanning line in a sense opposite to the bow in the line which has been produced by the hologon deflector, as long as there is distortion in the focusing lens.
It is not clear from this Kramer patent whether the deviation from straightness disclosed is theoretical or empirical, as the curves are smooth and no actual data is discussed. However, it can be seen in FIG. 5 that different degrees of bow are experienced over different field positions with different degrees of lens tilt. Bow seems to be optimized at a tilt of -8.5.degree. shown by curve No. 1, as both curves 2 and 3 have what appear to be greater bow at -8.0.degree. and -9.0.degree. respectively. In any event, the system requires the use of a tilted distortion containing focusing lens which may or may not be desirable in all applications and which is definitely expensive in any event.
This Kramer patent does not disclose that either barrel distortion or pincushion distortion do not have other inherent limitations which prevent the system from functioning as intended. It is also noted that the system shown requires a relatively precise alignment of the angle of the lens tilt with respect to the axis of the scan line. The Figures suggest that some variation in tilt angle results in less effective bow correction. The advantages of the system are stated to be that the previously needed equal angles of incidence and diffraction and the like set forth in Kramer U.S. Patent No. 4,289,371 are not needed with the tilted lens feature. The tilted lens system allows for use of other ratios of .lambda..sub.r /d and other incident angles, to provide freedom for the design of the hologon scanner. Nowhere is it stated that results are obtained which are better than the theoretical 0.0016 inch.
Finally, additional improvements in hologon deflector systems are disclosed in Kramer U.S. Pat. No. 4,973,112. The principal feature of this Kramer patent is to employ dispersive optical elements such as gratings and prisms to correct for deviations from straightness in the scan line which are caused by the inherent bowing of the scan line by rotating hologon deflector discs. Other errors and problems in the system are also corrected by the newest Kramer patent system.
In Kramer U.S. Pat. No. 4,973,112, several embodiments are disclosed which are suggested to solve several different problems. Bow is reduced by an auxiliary plane grating placed in the path of the scan beam exiting the hologon. The grating lines in the auxiliary grating are parallel to the hologon facet grating lines for the hologon corresponding to the center of scan to produce a diffracted, deflected beam. In discussing the effect of the auxiliary grating on bow, Kramer suggests that a scan line bow is produced by the hologon because the geometric relationship of the grating lines and the scan beam changes as the hologon rotates. The auxiliary grating element bows the scan line as well, Kramer suggests, because the incident beam changes its angular orientation with respect to the normal to the grating surface as it scans across the surface. Thus it is suggested that when the auxiliary grating is tilted with respect to the plane in which the plane grating hologon rotates, significant scan line bow correction is achieved.
Another system is disclosed in Kramer U.S. Pat. No. 4,973,112, in FIG. 2, in which an auxiliary plane grating is used in combination with a prism which is designed to magnify the scan beam bow produced by the hologon. Thus the auxiliary plane grating compensates for both the bow caused by the hologon rotation and also for the bow which is magnified by the prism. The function of the prism, therefore, is not to correct bow but rather to eliminate or reduce an ellipticity problem as described in the discussion relating to FIG. 2. Unfortunately, there is nothing in this reference which specifically shows how much bow has been reduced quantitatively beyond that which has been already shown in the art. In point of fact, there is no reason to expect that any improvement in bow reduction is found since all of the calculations are theoretical and are not based upon experimental results.
One particular application of this technology is in systems wherein the laser beam is being applied to the internal surface of a drum, such as in a laser printer or copier or the like. There is no system presently available in which the entire series of elements rotate together as a unit, where the ideal locus of point image would be a perfect circle. Wobble, or scanner rotation in the y-z plane prevents an effective design at the present time.
Accordingly, it is an object of this invention to provide a device for minimizing bow in scan beams produced by hologons and the like.
Similarly, it is an object of this invention to provide a device for minimizing wobble in scan beams produced by hologons and the like.
Another object of this invention is to provide a device for substantially eliminating bow as described by at least an order of magnitude over a system without such a device.
Similarly again, another object of this invention is to provide a device for substantially eliminating wobble by at least an order of magnitude over a system without such a device.
Yet another object of the present invention is to provide a bow reducing device which does not depend on lenses which have distortion and which are tilted at an angle with respect to the path of the scan beam.
Still another object of the present invention is to provide a device which is simple to use in hologon systems and which is effective over wide ranges of laser wavelengths and hologon grating distances.
Other objects will appear hereinafter.